eina-cxx: remove a warning.

Summary: When assert is disabled 'diff' variable was unused.

Reviewers: tasn, cedric

CC: JackDanielZ, smohanty, felipealmeida, cedric

Differential Revision: https://phab.enlightenment.org/D893

Signed-off-by: Cedric Bail <cedric.bail@free.fr>

src/bindings/eina_cxx/eina_inarray.hh

@@ -556,6 +556,7 @@ public:
           new (&*first++) T(t);
         std::size_t diff = last - first;
         assert(diff == _array->len - index - n);
+        static_cast<void>(diff);
         while(first != last)
           {
             new (&*first++) T(*old_first);